Optical characterisation techniques are frequently used for characterising materials, such as e.g. in material characterisation, medical applications, molecular diagnostics, chemical characterisation, etc. The latter typically may be used for characterising molecules, cells or small particles in small liquid samples, based on the measurement of light absorption and/or fluorescence in the sample. Besides the physical characteristics of the sample, the obtained optical signal typically also depends on the optical path length of the illumination beam in the liquid sample. The latter information typically is required for performing accurate measurements of e.g. the concentration of a particle in a liquid sample. In a large number of today's applications, the amount of fluid material typically available for performing characterisation is limited.
One approach for optically characterising small amounts of fluids is described in International patent application WO 01/14855. A liquid sample is held between two moveable parts comprising optical fibres by surface tension and optical characteristics can be measured by illuminating the sample fluid and collecting a luminescence response thereof. The technique typically requires direct contact between the sample and the read-out unit, implying that the measurement may be disturbed by contaminations originating from previous measurements. Furthermore, due to the lack of a suitable containing structure around the sample fluid, the free air-liquid surface is large and therefore evaporation of the liquid sample can occur on a short time scale.
It is known that small reservoirs, e.g. with a diameter in the order of 1 millimetre or smaller can be used to contain small liquid samples. However, filling of small reservoirs with liquids and control thereof is difficult due to the need for pressure to introduce the fluid in the small reservoir.